The invention relates to a modular force-measuring cell for a weighing scale and it also relates to a weighing scale with a modular force-measuring cell.
Known force-measuring cells of weighing scales are equipped with a force transducer that is on one side connected to a scale housing serving as support base and on the other side to a weighing pan carrier through which the force to be measured is introduced. Force transducers can be configured in a variety of ways. Widely used are transducer designs with an elastically deformable body as a core element, or with a device that performs a force compensation, in most cases by means of a counterbalancing force which is generated by a current-regulated electromagnet and is acting through a lever mechanism.
To give an example, a force transducer which is described in [1], DE 199 39 633 A1, and which is referred to as “counterforce” or “force receiver” in [2], EP 0 670 479 A1, has an elastically deformable body connecting a housing-connected fixed part of the transducer to a force-application part or, in the case of a weighing scale, to a weighing-load application part. The force transducer has transverse grooves at the transitions between the deformable body and the parts that serve to connect the force transducer to the scale housing and the weighing pan carrier. The transverse grooves serve to mechanically uncouple the deformable body in which the deformations effected by the applied forces are measured by means of sensors, such as by means of strain gauges.
The deformable body can be configured as a parallelogram-shaped measuring element with an arrangement of guide members resembling a parallelogram (see for example [3], EP 0 511 521 A1).
The analog signal representing the measurement is generated by means of strain gauges that can be connected to each other in a bridge circuit. The signal can be digitized in a converter circuit and subsequently put through further processing steps. The principal structure of a bridge circuit with strain gauges is described, e.g., in [4], U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik, 11th edition, first reprint, Springer Verlag, Berlin 1999, pages 1242–1243.
A weighing device with a force transducer, a converter device attached to the force transducer, and with a circuit arrangement is described in EP 0 319 176 A [6]. The force transducer and the aforementioned associated elements are enclosed together in a compact unit. The circuit device communicates through a plug connector with the electronic elements of the weighing device. The circuit device includes a device for producing a digital representation of a force acting on the load receiver, a device applying at least one correction factor to the digital representation, and a device for delivering an output of the digital representation. Thus, the enclosed force transducer can be adjusted as a unit and installed in a weighing device. However, this solution has the disadvantage that the analog and digital signal processing elements are placed in immediate proximity of the force transducer. The temperature effects resulting from the heat dissipation of the circuits can be difficult to manage, which complicates the temperature compensation process. Furthermore, the production of these force transducers is expensive, and in case of a failure the entire unit including the electronic portion has to be replaced.
To support the further processing of the digitized measuring signal, the measuring cell described in [2] has a memory module in which characteristic parameters are stored that are specific to the individual measuring cell and are used for the correction of the measuring signals.
As described in [5], Patent Specification GB 1 462 808, the aforementioned correction applies in particular to errors that are caused by non-linearities, hysteresis phenomena, temperature and creep effects. The calibration- and compensation data required for the correction are determined during production at the factory through specific test and measuring procedures and are stored in the memory module (also see [1]).
In the modular force-measuring cell disclosed in [2], the memory module that serves to store the compensation data is arranged in fixed connection with the force transducer and spatially separated from a local circuit arrangement, so that four components, i.e., the strain gauge bridge, a temperature sensor, the force receiver, and the memory module together form a modular unit. The compensation data and correction factors depend primarily on the measuring signals of the temperature sensor that is likewise permanently attached to the force transducer. Appropriate components and methods for the measurement of temperature are described in [4], pages 1224–1231.
The memory module in the modular force measuring cell described in [2] is connected by means of a flexible flat ribbon cable to a local circuit arrangement that is thermally uncoupled from the force transducer and includes an A/D converter and a processor.
The concept of integrating the aforementioned components in a modular unit leads to a simplification in the servicing of a scale in which the modular unit is used. In case of a malfunction, the modular force-measuring cell can be separated from the local circuit arrangement and replaced. The local circuit arrangement, likewise, can be replaced without the need for a recalibration of the scale which in some cases would require the scale to be returned to the factory.
However, the installation of the memory module on the force transducer with the connection to the local circuit arrangement by way of the flexible flat ribbon cable involves some manufacturing costs. Added to this is the risk that the memory module will have a thermal effect on the force transducer during operation, which has to be taken into account in the compensation procedure.
Associated with a modular design configuration is the desire that after installation of a measuring cell into a scale, the latter is already optimally adjusted. In addition to this desirable trait, there are the constant requirements to increase the precision of the scale, to reduce the measuring errors that need to be corrected, and to achieve an even more accurate adjustment.